By reason of their structure, fountain pens of conventional design are inevitably liable to leak appreciable amounts of ink when exposed to fluctuations in temperature or pressure.
In the main, ink leaks may be caused as a result of the pen barrel being warmed by the hand of the user when writing, and during air travel, as a consequence of pressure dropping in the aircraft cabin. In either case, a difference in pressure is created between the inside of the reservoir (or cartridge) and the outside, sufficient to overcome the resistance of the ducts in the nib and cause the ink to escape.
In effect, the force exerted on the ink is due to the pressure of the gas (typically air) locked in the cartridge being greater than that of the surrounding environment, and will tend to continue expelling the ink until such time as the pressure of this same gas and the pressure externally of the cartridge have equalized.
With regard in particular to a drop in ambient pressure, it has been found that the problem persists even when adopting a hermetic cap, since the moment the cap is removed, the ink will in any event escape and leave self-evidently unwelcome stains, often indelible. In this situation, consequently, the fountain pen becomes unusable at least temporarily, until the cause of the variation in temperature and/or pressure disappears.
According to U.S. Pat. No. 2,340,359 (Ziegler) in a pen with an elastically compressibile sac structure as reservoir for the ink to avoid the expulsion of ink in case of changes in atmospheric pressure it suggested that the sac is equipped with a shrounding which is formed of one or more strands of filament wound closely about the sac, so to realize a net, such strand or filament being not extensible so to avoid any dilatation of the sac in case of changes in atmospheric pressure.
It has been found that the said solution is difficult to be realized in practice considering what it is described in the specification at page 2.
According to WO 2009/141747 it is provided a fountain pen with a refillable ink reservoir deformable between a first limit position with maximum internal volume and a second position with reduced internal volume and subject to the actions of a mechanism to create a vacuum inside the ink reservoir by compressing and decompressing the ink reservoir in view to consent the refilling of ink inside the reservoir. The fountain pen is equipped with a cap for covering the nib of the pen, when the pen is not being used, and it is equipped also with valve means provided to close the channel of passage of the ink from the reservoir and the nib with the cap in a position to cover the nib, so to avoid the expulsion of ink in case of changes in atmospheric pressure.
It has been found that in case of changes in atmospheric pressure when the cup is later removed the risk that a minimum flow of ink is expelled from the nib cannot be avoided due the structure of the ink compressing reservoir.